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Abstract Seagrass meadows perform an important ecological function as filters for incoming nutrients from surrounding watersheds, especially nitrogen (N). By enhancing N removal processes, including N burial in sediments and denitrification, seagrass meadows improve water quality. With accelerating losses of seagrass meadows worldwide, seagrass restoration plays a key role in reestablishing these coastal ecosystem functions. However, few measurements exist of N burial rates in temperate seagrass meadows and none have been published for restored meadows. In this study, we measured N burial rates in a large (6.9 km2) restored eelgrass (Zostera marina) meadow and compared N removal through burial to previous measurements of removal via denitrification. We also compared N removal to inputs from external loading and fixation and to N assimilation in seagrass biomass. We found that, in this meadow, burial was the dominant process of N removal; the burial rate of 3.52 g N m−2yr−1was comparable to rates in natural meadows within 10 yr after seeding and was more than 20× the rate in adjacent bare sediments (0.17 g N m−2yr−1). We also found that the high rates of N assimilation (2.62 g N m−2yr−1) created a substantial though temporary sink for nitrogen during the growing season. Our results highlight how seagrass meadows mediate N cycling through high rates of burial, which to date has been understudied in the literature. The successful return of the N filter function after restoration, shown here for the first time, can motivate continued efforts for seagrass restoration and conservation.
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Nitrogen fixation associated with epiphytes on the seagrass Zostera marina in a temperate lagoon with moderate to high nitrogen loads
Abstract As part of a long-term study on the effects of nitrogen (N) loading in a shallow temperate lagoon, we measured rates of N2fixation associated with seagrass (Zostera marina) epiphytes during the summer from 2005 to 2019, at two sites along a gradient from where high N groundwater enters the system (denoted SH) to a more well-flushed outer harbor (OH). The data presented here are the first such long-term N2fixation estimates for any seagrass system and one of the very few reported for the phyllosphere in a temperate system. Mean daily N2fixation was estimated from light and dark measurements using the acetylene reduction assay intercalibrated using both incorporation of15N2into biomass and a novel application of the N2:Ar method. Surprisingly, despite a large inorganic N input from a N-contaminated groundwater plume, epiphytic N2fixation rates were moderately to very high for a seagrass system (OH site 14-year mean of 0.94 mmol N m−2 d−1), with the highest rates (2.6 mmol N m−2 d−1) measured at the more N-loaded eutrophic site (SH) where dissolved inorganic N was higher and soluble reactive phosphorus was lower than in the better-flushed OH. Over 95% of the total N2fixation measured was in the light, suggesting the importance of cyanobacteria in the epiphyte assemblages. We observed large inter-annual variation both within and across the two study sites (range from 0.1 to 2.6 mmol N fixed m−2d−1), which we suggest is in part related to climatic variation. We estimate that input from phyllosphere N2fixation over the study period contributes on average an additional 20% to the total daily N load per area within the seagrass meadow.
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- PAR ID:
- 10468232
- Publisher / Repository:
- Springer Science + Business Media
- Date Published:
- Journal Name:
- Biogeochemistry
- Volume:
- 166
- Issue:
- 3
- ISSN:
- 0168-2563
- Format(s):
- Medium: X Size: p. 211-226
- Size(s):
- p. 211-226
- Sponsoring Org:
- National Science Foundation
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